In order to throw some light on the rheological behavior of the upper mantle of the Earth, a detailed series of high-temperature deformation experiments has been performed on olivine single crystals at 1 atm pressure under highly controlled thermodynamic conditions. The effects of stress, temperature, oxygen fugacity and orthopyroxene activity on the deformation rate of olivine have been carefully measured for the major high-temperature slip systems. These experimental results have been extrapolated to the pressures and strain rates of the upper mantle to provide new insight into the mechanical behavior of the mantle and a better constrained approach to extrapolation of laboratory deformation data to the conditions present in the upper mantle. We suggest that extrapolations in stress provide the most reliable means for extending laboratory creep data to uper mantle strain rates. We also predict that, in the dislocation creep field, the bulk of the strain in olivine in the upper mantle is accomodated by the (010)[100> slip system. In addition, true mantle strengths are probably most accurately modelled using the experimental data for samples oriented favoring the (010)[001> slip system. However, the reliable prediction of mantle strengths requires highly accurate temperature versus depth relationships and good indications of the local oxygen fugacity in the mantle. ¿ American Geophysical Union 1990